The present invention relates generally to welding fixtures and more particularly to a fixture for positioning grid straps in an egg-crate array during their welding together into a nuclear fuel rod spacer grid.
Nuclear fuel assemblies include a matrix of nuclear fuel rods which are arrayed in rows and columns and which are held in the desired configuration by a plurality of fuel rod grids. These grids are produced from "straps" which are linearly extending, generally rectangular elements, characterized by having slots extending from one edge approximately halfway through the depth of the strap. The grid straps are assembled so that one strap is in mating relationship with the other strap. Thus, the slot of one strap engages the other strap at a portion thereof which is in alignment with the slot of that other strap, with the result that the grid is of the same depth as each of the straps which forms the grid. The resulting grid has a first set of straps which are substantially parallel to each other, and equally spaced, and a second set of straps which are parallel to each other and equally spaced, the straps of one set being perpendicular to the straps of the other set. All of the aforesaid straps are designated as "inner grid straps", and they are placed in mating relationship to form a square grid of square cells, in the above noted rows and columns. In addition, there are provided outer grid straps, which are placed on the four sides of the grid.
The assembled grid straps are held in a fixture and welded together. The welding fixture must insure the parallel and equal spacing of the grid straps of each set of grid straps, the perpendicularity of the grid straps of each set relative to the grid straps of the other set, and that the edges of the grid straps are straight. Further, the use of welding beam techniques requires not only the holding of the grid straps as hereinabove described, but also requires that the laser beam have access to each of the locations to be welded, particularly to the points of intersection of the grid straps.
An existing grid welding fixture used with laser beam welding, which has been in use for more than one year by the inventor's assignee (see U.S. patent application No. 414,265 entitled Welding Plates For A Fuel Rod Grid, by Richard M. Kobuck et al., which was filed 9/1/82), uses clamping bolts to releasably secure a top welding plate to a bottom welding plate. The welding plates are thick for required rigidity. Each welding plate is expensively machined to provide alignment grooves for the grid strap edges and to provide access holes for the laser welding of the grid straps at their intersection points. What is needed is a lighter and less expensive grid welding fixture.